The present invention relates generally to continuously optically pumped, solid-state, repetitively-pulsed lasers. It relates in particular to stabilization of thermal-lensing in a solid-state gain-medium in a frequency-multiplied, repetitively-pulsed laser.
Optically-pumped frequency-multiplied, repetitively-pulsed lasers are finding increasing use in laser material processing operations such as precision micromachining, marking, stereo lithography, and hard-disk texturing. One preferred laser type for this purposes is an extracavity frequency-tripled, pulsed, solid-state laser including a solid state gain-medium, such as Nd:YAG or Nd:YVO4, which provides fundamental radiation at a wavelength of about 1.064 micrometers (xcexcm). A detailed description of one example of such a laser can be found in U.S. Pat. No. 5,912,912 (Caprara et al.) assigned to the assignee of the present invention.
Such a laser is often designed to operate in a continuous repetitive pulsing mode at a selected pulse-repetition rate. In laser processing operations, however, the laser may be caused to deliver bursts or trains of pulses at this pulse-repetition rate for processing, with intervals between bursts when no processing occurs. The duration of such bursts depend on the processing operation. Intervals between bursts may vary, for example, according to time required to move a processing beam from one location to another on a workpiece being processed. It has been found that for short bursts of pulses, if the duration of the burst is not much greater, if at all, than the time required for the solid-state gain-medium to reach a thermal-lensing equilibrium a substantial change of thermal-lensing occurs during all, or some major portion, of a burst. Such a thermal-lensing change can lead to a variation in peak power of laser pulses and laser mode properties over the duration of the burst, which, in turn, can lead to imprecise processing operations.
Thermal-lensing is due to a spatial variation in refractive index of the solid-state gain-medium resulting from a thermal gradient in the gain-medium. This thermal gradient results, among other factors, from heating of the gain-medium by a portion of pump-light light power absorbed therein which is not extracted as laser radiation. Accordingly thermal-lensing is a function of, among other factors, pump-light power delivered to the solid-state gain-medium, and energy extracted from the gain-medium as laser radiation. In a repetitively pulsed laser, this extracted power is, in turn, dependent on the pulse-repetition rate. As described in the Caprara et al. patent, a laser-resonator can be variably configured to be adjustable for accommodating a range of equilibrium thermal-lensing effects resulting from operating the resonator at different powers and pulse-repetition rates. In operating such a laser to deliver pulse-trains or bursts (burst mode operation), a thermal-lensing change can occur as a result of a transition from a condition where no pulse is being delivered to a condition where a burst of pulses is delivered. This can occur either before a laser processing operation begins or from one pulse-burst to the next.
When no pulse-burst is being delivered, the gain-medium remains continuously pumped, but laser action is inhibited by operating an optical switch (Q-switch) which introduces a variable of controlled loss in the resonator of the laser. In one example, a Q-switch is arranged to be driven (rapidly opened and closed) during delivery of a pulse-burst by a modulated voltage applied to an acousto-optic or electro-optic crystal located in the resonator. The modulation frequency of which establishes the pulse-repetition rate.
When no pulses are being delivered, a maximum proportion of the pump-light power contributes to thermal-lensing. When a pulse-burst is being delivered some proportion of the pump-light power is extracted from the gain-medium as laser radiation. This reduces the temperature in the gain-medium and, correspondingly the thermal-lensing. This temperature-reduction occurs progressively over a time-period depending on the thermal inertia of cooling the gain-medium.
Because the resonator is adjusted to compensate for thermal-lensing when the rod reaches an equilibrium temperature (equilibrium thermal-lensing), the output power will progressively increase, once pulsing is initiated, reaching a maximum at equilibrium. Because of this, laser beam parameters such as mode-size and divergence and peak pulse-power will vary for at least an initial portion of the burst duration. This problem will be exacerbated when pulse-bursts are of different duration or have different durations therebetween, as may be required in a sequence of laser-processing operations. If pulse bursts are of shorter duration than time required to reach equilibrium the peak pulse power therein may fall short of the maximum possible for the resonator.
There is a need for a method of overcoming this peak power variation problem during a burst or repeated bursts of pulses.
The method of the present invention is directed to a method of operating a laser for performing a laser processing operation. The laser has a laser-resonator including an optically-pumped gain-medium. One or more optically-nonlinear crystals are located outside the resonator for converting laser radiation delivered by the resonator into frequency-converted radiation. The peak power of the frequency-converted radiation is dependent on delivery parameters of the laser radiation from the laser-resonator. The gain-medium exhibits a thermal-lensing effect on being optically pumped. The laser-resonator is configured to compensate for a predetermined range of the thermal-lensing. In one aspect, the method of the present invention comprises a first step of operating the laser to deliver laser radiation with delivery parameters thereof being such that frequency-converted radiation generated therefrom has insufficient peak power to perform the laser processing operation. In a second step, following this first step, the laser is operated to deliver laser radiation in the form of a train of pulses with delivery parameters thereof being such that pulses in a train of pulses of frequency-converted radiation generated therefrom have sufficient peak power to perform the laser processing operation. These may be defined as processing pulses. During the first and second steps, the laser is operated such that optical pumping power and average power of the laser radiation provide that the thermal-lensing effect is within the predetermined range for which the laser resonator is compensated.
Providing that the thermal-lensing effect is within the predetermined range during the first and second step ensures that there is no significant thermal-lensing change between intervals when processing pulses are being delivered and intervals when processing pulses are not being delivered. By this arrangement, at least the second and all other processing pulses in a train thereof have about the same peak power. By controlling loss in the resonator with a delay time before the initiation of a processing pulse train, i.e., between the first and second steps, the first pulse in the pulse-train can be controlled such that all processing pulses in the pulse-train have about the same power. Accordingly, the method of the present invention avoids the gradual increase in peak power that is experienced in pulse trains or bursts delivered by prior art lasers.
The above described first and second steps can be repeated such that frequency-converted radiation is generated as a sequence of trains of pulses having sufficient power to perform the processing operation having intervals therebetween in which insufficient frequency converted power is generated for the processing operation. Accordingly, the method of the present invention provides that the peak power of pulses in the trains of processing pulses can be maintained essentially constant independent of the duration of the trains of pulses.
In the first step, laser radiation may be delivered as a train of pulses having a higher pulse-repetition rate than that of the processing pulses in the second step. Alternatively, in the first step laser radiation may be delivered as continuous-wave (CW) radiation. Both of these options result in a lower peak power of laser radiation in the first step than the peak power of laser radiation in the second step.
In another aspect of the present invention, laser radiation delivered in the second step may be used directly for a processing operation for which the wavelength of the laser radiation is appropriate and which has a threshold for the operation which is dependent on peak power of the laser radiation. In this aspect of the method of the present invention, the first step includes operating the laser to deliver laser radiation having insufficient peak power to perform the laser processing operation. In a second step following this first step, the laser is operated to deliver laser radiation in the form of a train of pulses having sufficient peak power to perform the laser processing operation. During the first and second steps, the laser is operated such that the optical pumping power and average power of the laser radiation provide that the thermal-lensing effect is within the predetermined range. During the first step the laser radiation may be provided as CW radiation or as a train of pulses having a higher pulse-repetition rate than that of the processing pulses in the second step.
The method of the present invention is applicable to any laser processing operation that has a threshold power below which the processing operation can not be performed, and which would benefit from being performed by trains of pulses of equal peak power. Examples of such operations include but are not limited to precision micromachining, marking, stereo lithography, and hard-disk texturing.
One precision laser micromachining task may include drilling a series of holes specified to be identical in diameter and depth in a homogeneous material. A simple approach to this task involves using an equal number of pulses of equal energy at each hole site. Laser marking involves using a laser beam to form a plurality of spots over a surface. The spots my be arranged in the form of a detailed design or text, the quality of which is dependent on control of spot size. Stereolithography involves using a pulsed laser to selectively polymerize small volumes of a liquid plastic material to build up solid models. This may is done by rastering a focussed laser beam rapidly back and forth through the liquid plastic material while alternately turning the laser on an off to selectively expose only those volumes of the liquid required to build up a model. In hard disk texturing laser pulses are used to generate a plurality of bumps on the surface of a nickel phosphide coated disk. The bumps form a roughened area on the disk where read/write heads are xe2x80x9cparkedxe2x80x9d when the disk is not in use. Because close clearances are involved between the read/write heads and the disk, it is particularly important that all bumps have a precisely controlled height.
The above discussed applications represent only a sample of material processing applications which would benefit from advantages offered by the method of the present invention. From this summary and the detailed description of the inventive method set forth below, those skilled in the art will recognize other such applications for which the inventive method would offer advantages without departing from the spirit and scope of the invention.